The standard for integrating chip-to-chip communication is the printed circuit board. Printed circuit boards (PCBs) are used to interconnect and assemble electronic circuits. A typical PCB includes at least a resin-based material, a reinforcement material, and a conductive foil. By etching traces between integrated circuits (ICs) disposed upon them, PCBs provide electrical conductor paths between the ICs. PCBs also provide mechanical structure for the components that make up the system.
By far the most common PCB material is a fiber-reinforced glass epoxy material, known in the industry as Fire Retardant-4, or FR4. Woven fiberglass, impregnated with an epoxy resin, provides a solid, yet adaptable, material upon which the ICs can be disposed. The traces etched upon or within the PCB, typically copper, are intended to provide the sole signal path between circuits. Electrical signals, however, do not always follow the intended path.
One of the measured characteristics of the PCB is its dielectric constant. The dielectric constant of a material relates to the velocity at which signals travel within the material. The speed of a signal propagating along a trace is inversely proportional to the square root of the dielectric constant of the PCB upon which the trace is formed. Thus, the dielectric constant of the PCB affects the speed of all signals propagating on the PCB. The dielectric constant is actually variable, and may change with a modification in frequency, temperature, humidity, and other environmental conditions. Further, because the PCB is heterogeneous, comprising woven strands of fiberglass embedded in an epoxy resin, the dielectric constant at any point on the PCB is likely to vary. Thus, while the signal may follow the path of the trace, there may be some loss due to the changing dielectric constant of the underlying PCB. For very high-speed signals, the loss may be unmanageable.
Another characteristic relevant to signal transmission is the dissipation factor of the PCB. Dissipation factor is a measure of the electrical losses in a material. Materials may have similar dielectric constants, yet have very different dissipation factors. Particularly where high-speed signals are transmitted, the dissipation factor of the material, as well as its dielectric constant, are considered during system design.
Processor-based systems, such as personal computers, server systems, and the like, often include multiple PCBs connected together. A motherboard PCB may have connectors for receiving one or more daughtercards, for example. As the signal passes between the motherboard and the daughtercard, loss may occur because the two boards are not impedance-matched with each other, or because the connector is not impedance-matched with either the motherboard or the daughtercard. Impedance matching becomes more difficult as the signal speed increases.
Current high-speed interconnect technologies require a substantial amount of wiring between chips. For example, a single PCI Express connection has 16 lanes (two differential signal pairs traveling in opposite directions), requiring 64 wires between chips. (The PCI Express bus is a high-performance bus for connecting processors, add-in cards, controllers, and the like. The PCI Express Specification is available from The PCI Special Interest Group, Portland, Oreg. 97124.) To support this and other high-performance buses, PCBs may include many layers, employ increasingly sophisticated shielding techniques, and so on.
Additionally, signal speeds of up to 6.25 GigaTransfers/second (GT/s) are being achieved in many processor-based systems, with speeds exceeding 10 GT/s expected in the near future. Current FR4-based PCB materials are characterized by severe dielectric loss at these speeds. Other materials have been considered, to replace current PCB designs, but are prohibitively expensive.
Thus, there is a continuing need to provide an alternative to the current PCB model for providing high-speed interconnections between circuits.